The invention relates generally to time-division switching systems and more particularly to a telephone system for switching PCM (Pulse Code Modulated) data words among large numbers of time-division multiplex lines.
It is the function of a telephone switching system to establish communication connections between calling lines or trunks and called lines or trunks. Systems are known in the prior art in which analog signals on a plurality of lines or trunks are converted to PCM data words and are multiplexed onto a single transmission line having recurring time frames each comprising a plurality of channels. A channel is an identifiable time period on the time separated transmission line which occurs once in each time frame on the line. Known prior art systems typically have 24 channels per time frame and data words from 24 independent lines or trunks are transmitted during each time frame. In order to make maximum use of the available channels or conversely to minimize the time period of each channel, each data word occupies an entire channel. PCM data words may be switched among multiplex lines by selectively transferring PCM data words from the various channels on an input multiplex line to a predefined channel on one of a plurality of output multiplex lines. The transfer of data words from input multiplex lines to output multiplex lines may be accomplished by means of a space-division network which is reconfigured at a predetermined rate compatible with the rate at which the data is received from input multiplex lines. Typically such a switching network can be reconfigured and used to transmit data words once during each time period equal to the time period of a channel.
A space-division switching network which is utilized to transfer information among a large number of input and output multiplex lines must have a high capacity and low blocking. Such a switching network is typically physically large resulting in time delays of information transmitted therethrough and these delays vary in accordance with the particular path used for transmission through the network. The variability of transmission delays necessitates the concatenation of a start code to each data word before transmission to the network to permit asynchronous detection at the network outputs. This increases the amount of information each data word must convey. The previously mentioned reconfiguration rate requires that communication paths through each of the switches which make up network paths can exist for approximately the time period of a channel. Due to the variability of delay time, the effective time period for each point along the communication path is the channel time period minus a maximum delay in transmission. However, as previously stated, each data word when received by the switching arrangement occupies an entire channel time period.